For years I've had a Lionel 300 Hellgate bridge (MTH 11-90001) on my layout:
The topic of clearances has come up here on the forum a few times. I posted these photos in response to questions about the Lionel Super 381: It does NOT fit through the #300 Hellgate Bridge:
Note that the clearance problem is actually width of the bridge, not height.
Rumors have abounded about the Lionel re-issue of the bridge in 1999 as the number 305, modified to accept two O Gauge tracks (Lionel 6-32904).
I finally found a used one for sale.
The "original" 300 Hellgate Bridge is 28-1/2" long overall, 10-3/4" wide, and 11-1/2" high.
The clearances of the bridge opening are 5-7/8" wide and 7-1/2" high at the center point of the arch (7" above tubular track rail).
The re-issue 305 Hellgate bridge is 30" long overall, 12" wide, and 11-1/8" high.
The clearances of the opening are 7" wide and 7-1/2" high at the arch center point - still 7" above the track rails.
Here is the 305 installed on my layout in place of the 300 (involving minimal modification of the approaches to accomodate the extra 1-1/2" in length):
And here is the Super 381, under it's own power, clearing the bridge! (Note the obvious... the pantographs are down!!)
